1. Field of the Invention
The present invention relates to coating solutions for use in forming Bi-based ferroelectric thin films, and ferroelectric thin films, ferroelectric capacitors and ferroelectric memories formed with said coating solutions, as well as processes for production thereof. More particularly, the invention relates to coating solutions that lead to little leakage current, are capable of forming dense Bi-based ferroelectric thin films, are applicable to electrodes other than a Pt one, and have good keeping quality. The invention also relates to ferroelectric thin films, ferroelectric capacitors, and ferroelectric memories formed with such coating solutions, as well as processes for the production thereof.
2. Description of Related Art
Thin films of bismuth layer-structured ferroelectrics (BLSF) represented by the general formula (Bi.sub.2 O.sub.2).sup.2+ (A.sub.m-1 B.sub.m O.sub.3m+1).sup.2 - [where A is a mono-, di- or trivalent ion (as of Bi, Pb, Ba, Sr, Ca, Na, K or a rare earth element) or combinations of these ions; B is a tetra-, penta- or hexavalent ion (as of a metallic element like Ti, Nb, Ta, W, Mo, Fe, Co or Cr) or combinations of these ions; and m is an integer of 1-5] have recently been found to feature good characteristics such as requiring small coercive field in remanent polarization P-E hysteresis curves and hence experiencing less fatigue as a result of repeated polarization switching. This has spotlighted the potential use of BLSF thin films as materials for the fabrication of semiconductor memories and sensors (T. Takenaka, "Bismuth Layer-Structured Ferroelectrics and Their Grain Orientation" in Report of the Workshop on Applied Electronics Properties, The Japan Society of Applied Physics, pp. 1-8, Nov. 22, 1994; and "Ceramics", vol. 30, No. 6, pp. 499-503, 1995). Among the BLSF thin films so far reported, those of an SrBi.sub.2 Ta.sub.2 O.sub.9 system which are represented by the formula (Bi.sub.2 O.sub.2).sup.2+ (SrTa.sub.2 O.sub.7).sup.2- are of particular interest since the desired characteristics are conspicuous in them.
Such BLSF thin films can be formed by various methods including sputtering, CVD and coated film formation. However, due to the many metal oxides that have to be incorporated as constituents, sputtering and CVD techniques require costly apparatus and considerable difficulties are involved in controlling the composition of ferroelectric thin films at desired levels; hence, these techniques are not suitable for practical applications, particularly on large-diameter substrates. In contrast, the coated film formation technique does not need expensive apparatus and can deposit films at comparatively low cost; in addition, they provide ease in controlling the composition of ferroelectric thin films at desired levels. Therefore, the coated film process for the formation of BLSF thin films holds much promise for commercial use.
While several formulations have been proposed for use as coating solutions in the formation of BLSF thin films by the coated film process, two typical cases are those prepared by dissolving carboxylate (e.g., 2-ethylhexanate) of Sr and Bi and alkoxides of Ta in acetate esters (Proceedings of the 12th Ferroelectric Materials and their Applications meeting on May 24-27, 1995, Paper presented by Mitsubishi Materials Corporation, 24-TP-11, pp. 57-58; and "Jpn. J. Appl. Phys.", vol. 34, pp. 5096-5099, 1995) and those prepared by dissolving 2-ethylhexanate of Sr, Bi, Ta, Nb, Ti, etc. in xylene to form coating solutions of a metallo-organic decomposition (MOD) type (Proceedings of the 12th Ferroelectric Materials and their Applications meeting on May 24-27, 1995, Paper presented by Olympus Optical Co., Ltd. and Symetrix Corporation, 26-TC-10, pp. 139-140). However, these coating solutions have had various problems. First, the 2-ethylhexanate of the respective metal components has a long-chain (C.sub.8) organic group, so a large portion of the coating solution is occupied by the organic content and there is much loss in the coating weight due to the burning out of the organic component in the process of film formation consisting of the application of the coating solution, baking of the applied coating and crystallization and a porous film will result. In addition, the surface morphology of coating film is not satisfactory enough to provide for easy application to the fabrication of VLSI devices. Further in addition, in order to form a thin film using the applied coating, the applied coating has to be annealed at an elevated temperature of 800.degree. C. to ensure appropriate electrical characteristics thereof, however, this is problematic from the viewpoint of semiconductor fabrication process.
The prior art coating solutions have further problems. The long-chained metal carboxylate (metallic soaps) of monobasic acids which are commonly used in the coating solutions are generally slightly soluble in polar solvents and, hence, aromatic solvents such as xylene and toluene are used to prepare the coating solutions. However, the coating solutions using such aromatic solvents have to be stored in glass or metallic containers in order to ensure that the evaporating solvents will not be lost to the ambient atmosphere. On the other hand, glass and metallic containers have the disadvantage that metallic components will dissolve out into the coating solution and this is by no means desirable in the art of semiconductor fabrication which hates the contamination with metallic impurities. Under these circumstances, it is preferred to use polyethylene or polypropylene containers which release only negligible amounts of contaminating metallic impurities, which can be protected against mechanical shocks by simple handling procedures and which are less costly and it is desired to use solvents that will leak out of the plastic containers in negligibly small amounts.
The aforementioned aromatic solvents are also very toxic to humans and subject to increasingly rigorous regulations in the methods of use, management and so forth.
If the long-chained metal carboxylate of monobasic acids are replaced by short-chained metal carboxylate of monobasic acids, little solubility is achieved in practical organic solvents. Lower alkoxides of metals are soluble in several polar solvents but on account of the great tendency to be hydrolyzed with moisture in the air, they have only poor keeping quality and practically acceptable levels of reproduction cannot be achieved in the result of coating operations.
In recent years, Pt has been employed for a lower electrode as well as an upper electrode in a semiconductor memory (ferroelectric memory) utilizing ferroelectric thin films (PZT, etc.) including the aforesaid Bi-layered ferroelectric thin film (Bi-based ferroelectric thin film). For the purpose of improving the fatigue property and simplifying fine processing, it is a general tendency to use metals such as Ir, Ru, Rh, Re and Os other than Pt, and metal oxides thereof, especially Ir, Ru and conductive metal oxides such as IrO.sub.2 and RuO.sub.2, are attracting the general attention as materials suitable for the above-mentioned purpose.
However, the conventional Bi-based ferroelectric thin films are not excellent in crystallinity and electrical characteristics on the electrodes made of Ir, Ru, IrO.sub.2 or Ru0.sub.2 other than Pt, and have such problems as film fatigue resulting from repeated polarization switching and serious leakage current. There is therefore available no report of satisfactory properties in the use of electrodes made of materials other than Pt.
Under these circumstances, it has been strongly desired to develop a coating solution that is capable of forming highly dense films of good quality, with little leakage current, leading to small contents of organic components on forming a film, soluble in a practicable organic solvent, and is excellent in keeping stability, and further, achieving a Bi-based ferroelectric thin film excellent in various properties as a ferroelectric memory even with electrodes made of materials other than Pt.